1. Field of the Invention
The present invention relates to piezoelectric ceramics which are applied to a piezoelectric device such as a ceramic oscillator, a ceramic filter or a ceramic discriminator, for example, and more particularly, it relates to piezoelectric ceramics which are applied to a surface mounted type piezoelectric component, for which heat resistance is required.
2. Description of the Background Art
In general, piezoelectric ceramics which are mainly composed of lead titanate zirconate (Pb(Ti.sub.z Zr.sub.1-z)O.sub.3) are widely applied to a ceramic filter or the like. Further, there are proposed piezoelectric ceramics, to which small amounts of various additives are added fop improving piezoelectric characteristics thereof.
In particular, piezoelectric ceramics which are applied to a ceramic piezoelectric filter having a flat group delay time (hereinafter referred to as GDT) characteristic and small phase distortion must have a small mechanical quality factor Qm. As to such piezoelectric ceramics having-a small mechanical quality factor Qm, known are piezoelectric ceramics which are prepared by adding niobium oxide, antimony oxide or tantalum oxide to lead titanate zirconate (Pb(Ti.sub.z Zr.sub.1-z)O.sub.3), or that prepared by partially replacing Pb atoms of lead titanate zirconate with a rare earth element.
On the other hand, a material which is prepared by diffusing Mn in Pb(Zr.sub.0.52 Ti.sub.0.48)O.sub.3 is reported a piezoelectric ceramic containing a small amount of component diffused therein (M. Takahashi and S. Takahashi; Japan, J. Appl. Phys. Vol. 9, No. 8, pp. 1006 (1979)).
However, the aforementioned conventional piezoelectric ceramic having a small mechanical quality factor Qm is generally increased in piezoelectric constant d to be applied to an actuator, or mainly aimed at increasing an electromechanical coupling factor K for application to a broad-band filter. Thus, such piezoelectric ceramics generally have a low Curie temperature and insufficient heat resistance.
Some of the aforementioned conventional piezoelectric ceramics having small mechanical quality factors Qm have a high Curie temperature. When the temperature is increased in a soldering step or the like, however, the electromechanical coupling factor K is reduced to cause extreme displacement of resonance and antiresonance frequencies if electrodes provided across the piezoelectric ceramics are not short-circuited but opened.
When such a conventional piezoelectric ceramic having a small mechanical quality factor Qm is applied to a surface mounted type filter element, therefore, the filter characteristics are extremely deteriorated upon exposure to a high temperature of about 250.degree. C. in a reflow soldering step.
Further, the aforementioned material prepared by diffusing Mn in Pb(Zr.sub.0.52 Ti.sub.0.48)O.sub.3 is unsuitable for a filter element due to high temperature dependence of resonance and antiresonance frequencies.